The present invention relates to a device and a method for suppressing oscillations in voltage converters. In particular, the invention is applicable to generation of high voltage by means of so-called "flyback" converters.
In the field of voltage conversion, the use of so-called "flyback" converters is previously known, which converters can comprise a primary winding and a secondary winding on a common iron/ferrite core. The voltage conversion follows a periodic progression which starts with the connection of a certain input voltage to the primary winding during a certain time period. This gives rise to a linearly increasing current in the iron/ferrite core, in connection with the fact that the magnetic energy in the iron/ferrite core increases. Thereafter, the input voltage is disconnected, after which the current through the primary winding is cut off and the magnetic energy is transmitted to the secondary side, which causes a current to be generated in the secondary winding. Thereafter, this progression is repeated in a periodic manner.
The control of the input voltage and thereby also the current through the primary winding is carried out by means of a controllable switching element, preferably in the form of a transistor which is connected in series with the primary winding. The switching on and off of the transistor is controlled by means of a pulse circuit, wherein a voltage regulation can be carried out by controlling the frequency and/or the pulse width of the pulses which are generated in the pulse circuit. When the transistor is switched on, a current flows through the primary winding, and when the transistor is switched off the input voltage will consequently be disconnected.
The change of magnetic energy in the iron/ferrite core when a current is fed through the primary winding causes an induced voltage to be generated in the windings. Due to the fact that the windings normally consist of a number of turns of wire wound in several layers, with insulation between the layers, unwanted stray capacitances will be generated parallel to the windings. This gives rise to a voltage oscillation, which can be troublesome since the instantaneous voltage across the primary and the secondary windings may influence the subsequent cycle in the periodic progression. During conversion to high voltage, this is particularly annoying since the secondary winding has many turns of wire, which leads to large stray capacitances.
One way of correcting such a voltage oscillation in an inductor/transformer is to connect some type of suppression circuit across one of the windings. In high voltage converters, when the windings comprise many turns, the unwanted stray capacitances become large. Consequently, in order to suppress the oscillations described above it is required that the suppression circuit has a low impedance. This causes large losses, which is a drawback.
A voltage converter of the so-called "flyback" type is known from the document U.S. Pat. No. 4,616,300, in which converter the stray capacitances are utilized in resonance with the inductance of the transformer in order to increase the coefficient of efficiency. Due to the fact that this converter should be allowed to be synchronized with an external frequency, the primary side of the transformer is provided with a "switched damper", the purpose of which is to absorb resonant energy in order to simplify the synchronization. Thus, this document does not teach any device or method which relates to the problem according to the present invention, that is to prevent oscillations from occuring in the voltage control without giving rise to the losses which occur in a conventional suppression circuit.